1. Field of the Invention
The present invention relates to an organic electro luminescence device, and more particularly, to a dual panel type organic electro luminescence device and a fabrication method thereof.
2. Description of the Related Art
One of new flat panel display devices is an organic electro luminescence device. Because the organic electro luminescence device is a self-luminous display device, it has a high contrast and wide viewing angle compared to the liquid crystal display (LCD). Also, since the organic electro luminescence device does not require a backlight assembly, it is lightweight and slim. In addition, the organic electro luminescence device can decrease power consumption.
Further, the organic electro luminescence device can be driven at a low DC voltage and has a rapid response time. All the components of the organic electro luminescence device are formed of solid materials; thus, the device is endurable against external impact, can be used in a wide range of temperatures range, and can be manufactured at a low cost.
Specifically, the organic electro luminescence device is easily fabricated through a deposition process and an encapsulation process. Therefore, the fabrication method and apparatus of the organic electro luminescence device is simpler than those of an LCD or plasma display device (PDP).
Such a related art organic electro luminescence device is driven in a passive matrix mode that does not require separate switching elements.
In the passive matrix mode, scan lines and signal lines are crossed with one another and elements are arranged in a matrix. In order to drive pixels, the scan lines are sequentially driven according to time. Therefore, in order to produce a necessary mean brightness, the passive matrix organic electro luminescence device must provide instantaneous brightness corresponding to a product of a mean brightness and the number of lines.
In an active matrix mode, however, a thin film transistor (TFT), serving as a switching element to turn on/off pixel, is disposed in each sub-pixel. A first electrode connected to the TFT is switched on/off based on the sub-pixel, and a second electrode facing the first electrode is a common electrode.
In the active matrix, since a voltage applied to the pixel is charged in a storage capacitor (CST), a voltage must be applied until a next frame signal is input. Therefore, the organic electro luminescence device must be continuously driven during one picture regardless of the number of scan lines.
If the organic electro luminescence device is driven in an active matrix mode, uniform brightness can be obtained even when a low current is applied. Accordingly, the active matrix organic electro luminescence device has advantages of low power consumption, high definition, and large-sized screen.
FIG. 1 is a schematic sectional view of a related art bottom emission type organic electro luminescence device. In FIG. 1, only one pixel region including red, green and blue sub-pixels is illustrated for conciseness.
Referring to FIG. 1, first and second substrates 10 and 30 are arranged to face each other. Edge portions of the first and second substrates 10 and 30 are encapsulated by a seal pattern 40. A TFT T is formed on a transparent substrate 1 of the first substrate 10 in sub-pixel unit. A first electrode 12 is connected to the TFT T. An organic electro luminescent layer 14 is formed on the TFT T and the first electrode 12 and is arranged to correspond to the first electrode 12. The organic electro luminescent layer 14 contains light emission materials taking on red, green and blue colors. A second electrode 16 is formed on the organic electro luminescent layer 14.
The first and second electrodes 12 and 16 function to apply an electric field to the organic electro luminescent layer 14.
Due to the seal pattern 40, the second electrode 16 and the second substrate 30 are spaced apart from each other by a predetermined distance. Therefore, an absorbent (not shown) and a translucent tape (not shown) may be further provided in an inner surface of the second substrate 30. The absorbent absorbs moisture introduced from an exterior, and the translucent tape adheres the absorbent to the second substrate 30.
In the bottom emission type structure, when the first electrode 12 and the second electrode 16 are an anode and a cathode, respectively, the first electrode 12 is formed of a transparent conductive material and the second electrode 16 is formed of a metal having a low work function. In such a condition, the organic electro luminescent layer 14 includes a hole injection layer 14a, a hole transporting layer 14b, an emission layer 14c, and an electron transporting layer 14d, which are sequentially formed on a layer contacting with the first electrode 12.
The emission layer 14c has red, green and blue color filters for sub-pixels.
FIG. 2 is an enlarged sectional view of one sub-pixel region in the bottom emission type organic electro luminescence device shown in FIG. 1.
Referring to FIG. 2, a semiconductor layer 68, a gate electrode 62, and source and drain electrodes 80 and 82 are sequentially formed on a transparent substrate 1, thereby forming a TFT region. A power electrode 72 extending from a power line (not shown) is connected to the source electrode 80 and an organic electro luminescent diode E is connected to the drain electrode 82.
A capacitor electrode 64 is disposed at a lower portion with reference to the power electrode 72. The capacitor electrode 64 is formed of a same material as the semiconductor layer 68. A dielectric layer is interposed between the semiconductor layer 68 and the capacitor electrode 64. A region corresponding to them is a storage capacitor region.
Except the organic electro luminescent diode E, the elements formed in the TFT region and the storage capacitor region is an array elements A.
The organic electro luminescent diode E includes a first electrode 12, a second electrode 16, and an organic electrode luminescent layer 14 interposed between the first and second electrodes 12 and 16. The organic electro luminescent diode E is disposed in an emission region from which a self-luminous light is emitted.
In the related art organic electro luminescence device, the array element (A) and the organic electro luminescent diode (E) are stacked on the same substrate.
The bottom emission type organic electro luminescence device is fabricated by attaching the substrate, where the array element and the organic electro luminescent diode are formed, to the separate substrate provided for the encapsulation.
In this case, the yield of the organic electro luminescence device is determined by the product of the yield of the array element and the yield of the organic electro luminescent diode. Therefore, the entire process yield is greatly restricted by the process of forming the organic electro luminescent diode. For example, even though excellent array elements are formed, if foreign particles or other factors cause defects in forming the organic electro luminescent layer of a thin film of about 1000 Å thick, the corresponding organic electro luminescence device is defective.
Thus, there are loss of expense and material costs that are spent in fabricating the non-defective array element, resulting in the reduction of the yield.
In addition, the bottom emission type organic electro luminescence device has high stability and a high degree of freedom due to the encapsulation, but has limitation in aperture ratio. Thus, the bottom emission type organic electro luminescence device is difficult to apply to high-definition products. Meanwhile, in the case of the top emission type organic electro luminescence device, the design of the TFTs is easy and the aperture ratio is high. Thus, it is advantageous in view of the lifetime of the product. However, since the cathode is disposed on the organic electro luminescent layer, the selection of material is restricted. Consequently, the transmittance is limited and the luminous efficiency is degraded.